Radios generally function to receive broadcast signals. Usually, the user of a radio will not be interested in all of the signals that are broadcast on a communication resource (such as a particular frequency or a particular TDM time slot). Since many broadcast signals of interest include an identifying signal or other identifying indicia, many radios include a signal processor that examines the received broadcast signal to determine the presence of the identifying signal. If present, the radio can be further enabled to render the broadcast signal audible, visible, or otherwise as appropriate for that particular broadcast signal.
For example, some radios remain squelched unless a carrier can be sensed on a particular monitored communication resource. Other broadcast signals include special identifying signals, such as tone signals or subaudible digital signals, that the radio can recognize and respond to. Yet other signals of interest can be prefaced with a specific identifying preamble, such as an ID for the intended radio.
In order to expand the capabilities and flexibility of radios, digitizers and digital signal processors (DSPs) have been used. These processors typically receive digitized received signals. Further processing of the signal, equivalent to IF and discriminator processing, then occurs in the DSP in a digital manner. The resultant signal can then be converted into analog form and processed further as appropriate. For example, the resultant signal may be rendered audible in the case of a voice transmission.
DSPs, however, consume a significant amount of power when operating. This becomes a particular problem when seeking to use a DSP in a portable radio with limited power resources. DSP power consumption becomes of particular concern when the DSP operates both in the presence and absence of a broadcast signal of interest. Typically, the DSP must operate even in the absence of a broadcast signal of interest because the DSP itself aids in detecting the presence of a broadcast signal of interest. Unless the broadcast signals of interest occur at known times, the DSP must remain active in order to detect the signal when it occurs.